In recent years, powder coatings have become increasingly popular because these coatings are inherently low in volatile organic content ("VOC"), which significantly reduces emissions of volatile organic compounds into the atmosphere during application and curing processes.
Hydroxyl, carboxyl, carbamate and/or epoxy functional resins, such as acrylic and polyester resins having relatively high glass transition temperatures ("Tg"), are commonly used as main film-forming polymers for these coatings. The relatively high Tg of such acrylic polymer systems provides powder coatings having good storage stability. However, when exposed to the extreme temperatures which can be encountered during shipping and/or storage in many geographic areas, even better powder coating stability is desired.
Aminoplast resins are well known in the art as low cost crosslinking agents for hydroxyl, carboxyl and/or carbamate functional polymers in conventional liquid coating compositions. Common aminoplast resins are based on condensation products of formaldehyde with an amino- or amido-group carrying substance. Condensation products obtained from the reaction of alcohols and formaldehyde with melamine, urea or benzoguanamine are most commonly used in liquid coating compositions where they provide enhanced coating properties such as exterior durability, chemical resistance and mar resistance. Such aminoplast resins typically are in liquid form and, as such, generally are not suitable for use in curable powder coating compositions.
The alkoxylated aldehyde condensates of glycoluril, which are solid products, are the aminoplast resins most commonly employed as crosslinking agents in powder coating compositions. Although in solid form, these materials nonetheless can depress the Tg of the powder coating composition significantly, even when combined with high Tg film-forming polymers such as the acrylic polymers described above. Such a depression in Tg also can result in poor powder stability.
Moreover, the use of conventional aminoplast resins in curable powder coating compositions can result in the phenomenon commonly referred to as "gassing". "Gassing" occurs as a result of vaporization of the alcohol generated in the thermally induced aminoplast crosslinking reaction. The alcohol vapor is driven off through the coating film upon heating and, as the viscosity of the coating increases during the curing process, pinholes or craters are formed as gases escape through coating surface.
U.S. Pat. Nos. 3,904,623, 4,189,421 and 4,217,377 disclose a solid, non-gelled low molecular weight addition reaction product and a method for preparing the reaction product. The addition reaction product is suitable for use as a crosslinking agent in powder coating compositions when combined with polymers having various reactive functional groups. The crosslinking agent is the reaction product of 1.8 to 2.2 moles of a monohydroxy-, single-ring aromatic compound, for example phenol, and 1.0 mole of an alkoxymethyl aminotriazine compound, such as hexakis (methoxymethyl aminotriazine.
U.S. Pat. No. 4,393,181 discloses solid, adducts prepared from aminotriazine compounds and a large excess of polyhydric phenols. The adducts, due to their phenolic functionality, are useful as crosslinking agents for epoxy resins in powder coating compositions when used in conjunction with a curing agent accelerator such as an imidazole or benzimidazole.
U.S. Pat. No. 3,759,854 discloses heat-fusible powder coating compositions comprising a crosslinker prepared by pre-reacting a thermosetting polyester resin and a suitable conventional aminoplast crosslinking resin such as a condensation product of an aldehyde with melamine, urea or benzoguanamine.
U.S. Pat. No. 5,302,462 discloses a similar process for preparing a partially cured powder coating crosslinker. The crosslinker is prepared by partially reacting a less than stoichiometric amount of methoxymethyl aminotriazine with a linear, hydroxyl-terminated polyester.
U.S. Pat. No. 3,980,732 discloses a process for preparing a curable powder resin composition having a sintering temperature above 40.degree. C. The method comprises partially reacting a methylolamino compound with an aliphatic alcohol and an aliphatic diamide to produce an aminoplast condensate with a T.sub.g ranging from -10.degree. C. to 100.degree. C. and blending the aminoplast condensate with an acrylic or polyester resin having a glass transition temperature ranging from 60.degree. C. to 100.degree. C.
U.S. Pat. No. 4,185,045 discloses a powder coating composition comprising a solid crosslinking agent having a softening point ranging from 50.degree. C. to 120.degree. C. and prepared by heating 40 to 75% by weight of an acrylic polyol and 60 to 25% by weight of an alkoxyaminotriazine at 50.degree. to 120.degree., and a base resin having a softening point ranging from 60.degree. C. to 130.degree. C.
U.S. Pat. No.4,230,829 discloses a solid crosslinking agent having a softening point of 50.degree. C. to 120.degree. C. and prepared by heating 40 to 70% by weight of a polyester polyol and 60 to 30% by weight of an alkoxyaminotriazine.
While the above-described prior art aminoplast-based crosslinkers for powder coating compositions provide some improvement in "gassing" and powder stability over their liquid aminoplast counterparts, the powder coating compositions containing these crosslinkers can, nonetheless, exhibit some of the aforementioned deficiencies. In addition, most of the crosslinkers disclosed in the prior art are high molecular weight, partially cured and, hence, unstable mixtures. Further, the crosslinkers described in U.S. Pat. Nos. 3,904,623, 4,189,421, and 4,217,377 contain a significant amount of unreacted phenol impurity, which significantly limit their use in the powder coatings industry.
Thus, there remains a need for an aminoplast crosslinking agent suitable for use in curable powder coating compositions which provides a storage stable powder composition having the desirable coating properties usually associated with aminoplast-based liquid coatings without causing s coating surface defects due to "gassing".